1. Field of the Invention
This invention relates to the field of consumer electronics, and in particular to encryption techniques for copy and display protection for copyright material.
2. Description of Related Art
Digital recordings have the unique property that copies of the recorded material have the same quality as the original. As such, the need for an effective copy protection scheme is particularly crucial for the protection of copyright material that is digitally recorded. A number of protection schemes have been developed or proposed that rely upon a secure link between electronic devices. These schemes typically rely upon the manufacturers of the electronic devices to comply with an agreed upon standard to make the mass production of protected material economically infeasible. For example, a set top box may communicate a received broadcast to a receiving device, such as a VCR or television, in an encrypted form that is only decryptable by the intended receiving device. In like manner, a playback device, such as a VCR or DVD, communicates the material in an encrypted form that is only decryptable by its intended receiving device, and so on.
To effect this one-to-one encryption scheme, the proposed standards often call for the establishment of the encrypted link via a mutual key exchange. One such key exchange system is the “Diffie-Hellman” key-exchange algorithm, common in the art. FIG. 1 illustrates an example flow diagram for a key-exchange and subsequent encryption of content material using the Diffie-Hellman scheme. At 110, a first device, Device A, transmits a large prime n, and a number g that is primitive mod n, as a message 111 to a second device, Device B, that receives n and g, at 115. Each device, at 120 and 125, generate a large random number, x and y, respectively. At 130, Device A computes a number X that is equal to gx mod n; and, at 135, Device B computes a number Y that is equal to gy mod n. Device A communicates X to Device B, and Device B communicates Y to Device A, via messages 131, 136, respectively. Note that the determination of x from a knowledge of g and X, and y from a knowledge of g and Y, is computationally infeasible, and thus, an eavesdropper to the exchange of g, n, X, and Y will not be able to determine x or y. Device A computes a key K that is equal to Yx mod n, at 140, and Device B computes a key K′ that is equal to Xy mod n, at 145. Note that both K and K′ are equal to gxy mod n, and thus Device B knows Device A's key, and vice versa, while an eavesdropper to the exchange of g, n, X, and Y will not know the key, because the eavesdropper does not know x or y. This exchange of information from which to generate the knowledge of a corresponding key is termed a key exchange.
After effecting the key exchange, Device A encrypts the content material M 150 and communicates the encrypted material Ek(M) to Device B, at 160, via communications path 161. Because Device B's key K′ is identical to the key K that is used to encrypt the content material M 150, Device B uses key K′ to decrypt the received encrypted material Ek(M) to create a decrypted copy 150′ of the content material M 150, at 165. An eavesdropper to the communications path 161, not having a knowledge of the key K, is unable to decrypt the encrypted material Ek(M), and thus unable to create a copy of the content material M 150.
Note that the above described key exchange, and virtually all known secure key exchanges, require a bidirectional transfer of information between the devices. Many consumer electronic devices, however, are configured for a unidirectional transfer of content material. For example, as illustrated in FIG. 2, a conventional set-top box 210 or DVD player 230 includes a wide-bandwidth transmitter for transmitting 211, 212, 231, 232 the content material to a VCR 260 or display device 250, but rarely includes a receiver for receiving communications from the VCR or display device. In a device such as the VCR 260 that has bidirectional communications capabilities 251, 261, the key exchange can be effected by multiplexing the key exchange messages 111, 131, 136 of FIG. 1 onto the same paths 251, 261 that are used to communicate content material. Note, however, that the adoption of this multiplexing scheme for key exchange requires that the receiving device 250 contain receiving equipment that is compatible with the transmission 261 of content material. Thus, a conventional DVD player 230 that transmits 232 wide-bandwidth content material to the VCR 260 will be required to also contain a wide-bandwidth receiver to receive wide-bandwidth content-compatible transmissions 262 from the VCR to effect a key exchange, even though the DVD player will have no other practical use for this wide-bandwidth communications path 262. Note, also, that a switching means will be required at the VCR to redirect the wide-bandwidth output, from the conventional connection 261 to a display device 250, to the DVD player via this newly required communications path 262. Alternatively, additional transmitters, receivers, and communications connectors can be added to each consumer component 210, 220, 250, 260, etc. to effect the key exchange. Each of these options requires additional material and manufacturing costs to add the required communications and connection equipment.